Anode-Protecting Layer for a Lithium Metal Secondary Battery and Manufacturing Method

ABSTRACT

Provided is a lithium secondary battery, comprising a cathode, an anode, and a porous separator or electrolyte disposed between the cathode and the anode, wherein the anode comprises: (a) an anode active layer containing a layer of lithium or lithium alloy, in a form of a foil, coating, or multiple particles aggregated together, as an anode active material; and (b) a thin layer of a high-elasticity polymer, disposed between the anode active layer and the porous separator or electrolyte; the polymer having a recoverable tensile strain from 2% to 1,500%, a lithium ion conductivity no less than 10−6 S/cm (typically up to 5×10−2 S/cm) at room temperature, and a thickness from 1 nm to 10 μm, wherein the high-elasticity polymer contains a polyrotaxane network having a rotaxane structure or a polyrotaxane structure at a crosslink point of the polyrotaxane network.

FIELD OF THE INVENTION

The present invention relates to the field of rechargeable lithium metalbattery having a lithium metal layer (in a form of thin lithium foil,coating, or sheet of lithium particles) as an anode active material anda method of manufacturing same.

BACKGROUND OF THE INVENTION

Lithium-ion and lithium (Li) metal cells (including Lithium-sulfur cell,Li-air cell, etc.) are considered promising power sources for electricvehicle (EV), hybrid electric vehicle (HEV), and portable electronicdevices, such as lap-top computers and mobile phones. Lithium metal hasthe highest capacity (3,861 mAh/g) compared to any other metal ormetal-intercalated compound (except Li_(4.4)Si) as an anode activematerial. Hence, in general, rechargeable Li metal batteries have asignificantly higher energy density than lithium ion batteries.

Historically, rechargeable lithium metal batteries were produced usingnon-lithiated compounds having high specific capacities, such as TiS₂,MoS₂, MnO₂, CoO₂ and V₂O₅, as the cathode active materials, which werecoupled with a lithium metal anode. When the battery was discharged,lithium ions were dissolved from the lithium metal anode and transferredto the cathode through the electrolyte and, thus, the cathode becamelithiated. Unfortunately, upon cycling, the lithium metal resulted inthe formation of dendrites that ultimately caused unsafe conditions inthe battery. As a result, the production of these types of secondarybatteries was stopped in the early 1990's giving ways to lithium-ionbatteries.

Even now, cycling stability and safety concerns remain the primaryfactors preventing the further commercialization of Li metal batteriesfor EV, HEV, and microelectronic device applications. These issues areprimarily due to the high tendency for Li to form dendrite structuresduring repeated charge-discharge cycles or an overcharge, leading tointernal electrical shorting and thermal runaway. Many attempts havebeen made to address the dendrite-related issues, as briefly summarizedbelow:

Fauteux, et al. [D. Fauteux, et al., “Secondary Electrolytic Cell andElectrolytic Process,” U.S. Pat. No. 5,434,021, Jul. 18, 1995] appliedto a metal anode a protective surface layer (e.g., a mixture ofpolynuclear aromatic and polyethylene oxide) that enables transfer ofmetal ions from the metal anode to the electrolyte and back. The surfacelayer is also electronically conductive so that the ions will beuniformly attracted back onto the metal anode during electrodeposition(i.e. during battery recharge). Alamgir, et al. [M. Alamgir, et al.“Solid polymer electrolyte batteries containing metallocenes,” U.S. Pat.No. 5,536,599, Jul. 16, 1996] used ferrocenes to prevent chemicalovercharge and dendrite formation in a solid polymer electrolyte-basedrechargeable battery.

Skotheim [T. A. Skotheim, “Stabilized Anode for Lithium-PolymerBattery,” U.S. Pat. No. 5,648,187 (Jul. 15, 1997); U.S. Pat. No.5,961,672 (Oct. 5, 1999)] provided a Li metal anode that was stabilizedagainst the dendrite formation by the use of a vacuum-evaporated thinfilm of a Li ion-conducting polymer interposed between the Li metalanode and the electrolyte. Skotheim, et al. [T. A. Skotheim, et al.“Lithium Anodes for Electrochemical Cells,” U.S. Pat. No. 6,733,924 (May11, 2004); U.S. Pat. No. 6,797,428 (Sep. 28, 2004); U.S. Pat. No.6,936,381 (Aug. 30, 2005); and U.S. Pat. No. 7,247,408 (Jul. 24, 2007)]further proposed a multilayer anode structure consisting of a Limetal-based first layer, a second layer of a temporary protective metal(e.g., Cu, Mg, and Al), and a third layer that is composed of at leastone layer (typically 2 or more layers) of a single ion-conducting glass,such as lithium silicate and lithium phosphate, or polymer. It is clearthat such an anode structure, consisting of at least 3 or 4 layers, istoo complex and too costly to make and use.

Protective coatings for Li anodes, such as glassy surface layers of LiI—Li₃PO₄—P₂S₅, may be obtained from plasma assisted deposition [S. J.Visco, et al., “Protective Coatings for Negative Electrodes,” U.S. Pat.No. 6,025,094 (Feb. 15, 2000)]. Complex, multi-layer protective coatingswere also proposed by Visco, et al. [S. J. Visco, et al., “ProtectedActive Metal Electrode and Battery Cell Structures with Non-aqueousInterlayer Architecture,” U.S. Pat. No. 7,282,295 (Oct. 16, 2007); U.S.Pat. No. 7,282,296 (Oct. 16, 2007); and U.S. Pat. No. 7,282,302 (Oct.16, 2007)].

Despite these earlier efforts, no rechargeable Li metal batteries haveyet succeeded in the market place. This is likely due to the notion thatthese prior art approaches still have major deficiencies. For instance,in several cases, the anode or electrolyte structures are too complex.In others, the materials are too costly or the processes for makingthese materials are too laborious or difficult. Solid electrolytestypically have a low lithium ion conductivity, are difficult to produceand difficult to implement into a battery.

Furthermore, solid electrolyte, as the sole electrolyte in a cell or asan anode-protecting layer (interposed between the lithium film and theliquid electrolyte) does not have and cannot maintain a good contactwith the lithium metal. This effectively reduces the effectiveness ofthe electrolyte to support dissolution of lithium ions (during batterydischarge), transport lithium ions, and allowing the lithium ions tore-deposit back to the lithium anode (during battery recharge).

Another major issue associated with the lithium metal anode is thecontinuing reactions between electrolyte and lithium metal, leading torepeated formation of “dead lithium-containing species” that cannot bere-deposited back to the anode and become isolated from the anode. Thesereactions continue to irreversibly consume electrolyte and lithiummetal, resulting in rapid capacity decay. In order to compensate forthis continuing loss of lithium metal, an excessive amount of lithiummetal (3-5 times higher amount than what would be required) is typicallyimplemented at the anode when the battery is made. This adds not onlycosts but also a significant weight and volume to a battery, reducingthe energy density of the battery cell. This important issue has beenlargely ignored and there has been no plausible solution to this problemin battery industry.

Clearly, an urgent need exists for a simpler, more cost-effective, andeasier to implement approach to preventing Li metal dendrite-inducedinternal short circuit and thermal runaway problems in Li metalbatteries, and to reducing or eliminating the detrimental reactionsbetween lithium metal and the electrolyte.

Hence, an object of the present invention was to provide an effectiveway to overcome the lithium metal dendrite and reaction problems in alltypes of Li metal batteries having a lithium metal anode. A specificobject of the present invention was to provide a lithium metal cell thatexhibits a high specific capacity, high specific energy, high degree ofsafety, and a long and stable cycle life.

SUMMARY OF THE INVENTION

Herein reported is a lithium secondary battery, comprising a cathode, ananode, and an electrolyte or separator-electrolyte assembly disposedbetween the cathode and the anode, wherein the anode comprises: (a) alayer of lithium or lithium alloy (in the form of a foil, coating, ormultiple particles aggregated together) as an anode active material; and(b) a thin layer of a high-elasticity polymer having a recoverabletensile strain no less than 2%, a lithium ion conductivity no less than10⁻⁶ S/cm at room temperature, and a thickness from 1 nm to 10 μm,wherein the high-elasticity polymer is disposed between the lithium orlithium alloy layer and the electrolyte or separator-electrolyteassembly layer. The foil or coating of lithium or lithium alloy may besupported by a current collector (e.g. a Cu foil, a Ni foam, a porouslayer of nanofilaments, such as graphene sheets, carbon nanofibers,carbon nanotubes, etc.). A porous separator may not be necessary if theelectrolyte is a solid-state electrolyte.

High-elasticity polymer refers to a polymer, typically a lightlycross-linked polymer, which exhibits an elastic deformation that is atleast 2% (preferably at least 5%) when measured under uniaxial tension.In the field of materials science and engineering, the “elasticdeformation” is defined as a deformation of a material (when beingmechanically stressed) that is essentially fully recoverable uponrelease of the load and the recovery process is essentiallyinstantaneous (no or little time delay). The elastic deformation is morepreferably greater than 10%, even more preferably greater than 30%,further more preferably greater than 50%, and still more preferablygreater than 100%.

In some preferred embodiments, the high-elasticity polymer contains apolyrotaxane network having a rotaxane structure or a polyrotaxanestructure at a crosslink point of said polyrotaxane network.

Preferably, the rotaxane structure or polyrotaxane structure is selectedfrom rotaxane, a chemically modified rotaxane (rotaxane derivative), apolymer-grafted rotaxane, polyrotaxane, a co-polymer of polyrotaxane, agraft polymer of polyrotaxane, a polymer blend of polymer ofpolyrotaxane, a chemically modified polyrotaxane, or a combinationthereof.

In certain embodiments, the polyrotaxane network contains a polymerselected from polyethylene glycol, polypropylene glycol, polyethyleneoxide, polypropylene oxide, poly (succinic acid), an aliphaticpolyester, or a combination thereof.

This high-elasticity polymer layer may be a thin film disposed against alithium foil/coating layer surface or a thin coating deposited on thelithium foil/coating surface. It may be noted that lithium foil/coatinglayer may decrease in thickness due to dissolution of lithium into theelectrolyte to become lithium ions as the lithium battery is discharged,creating a gap between the current collector and the protective layer ifthe protective layer were not elastic. Such a gap would make there-deposition of lithium ions back to the anode impossible. We haveobserved that the instant high-elasticity polymer is capable ofexpanding or shrinking congruently or conformably with the anode layer.This capability helps to maintain a good contact between the currentcollector (or the lithium film itself) and the protective layer,enabling the re-deposition of lithium ions without interruption.

In certain embodiments, the high-elasticity polymer contains anelectrically conductive material (i.e. electron-conducting material)dispersed therein or mixed therewith. The electrically conductingmaterial may be selected from an electron-conducting polymer, a metalparticle or wire (or metal nano-wire), a graphene sheet, a carbon fiber,a graphite fiber, a carbon nanofiber, a graphite nanofiber, a carbonnanotube, a graphite particle, an expanded graphite flake, an acetyleneblack particle, or a combination thereof. The electrically conductingmaterial (e.g. metal nano-wire, nanofiber, etc.) preferably has athickness or diameter less than 100 nm.

The lithium salt dispersed in the high-elasticity polymer may bepreferably selected from lithium perchlorate (LiClO₄), lithiumhexafluorophosphate (LiPF₆), lithium borofluoride (LiBF₄), lithiumhexafluoroarsenide (LiAsF₆), lithium trifluoro-metasulfonate (LiCF₃SO₃),bis-trifluoromethyl sulfonylimide lithium (LiN(CF₃SO₂)₂), lithiumbis(oxalato)borate(LiBOB), lithium oxalyldifluoroborate (LiBF₂C₂O₄),lithium oxalyldifluoroborate (LiBF₂C₂O₄), lithium nitrate (LiNO₃),Li-Fluoroalkyl-Phosphates (LiPF₃(CF₂CF₃)₃), lithiumbisperfluoro-ethysulfonylimide (LiBETI), lithiumbis(trifluoromethanesulphonyl)imide, lithium bis(fluorosulphonyl)imide,lithium trifluoromethanesulfonimide (LiTFSI), an ionic liquid-basedlithium salt, or a combination thereof.

At the anode side, preferably and typically, the high-elasticity polymerfor the protective layer has a lithium ion conductivity no less than10⁻⁵ S/cm, more preferably no less than 10⁻⁴ S/cm, and most preferablyno less than 10⁻³ S/cm. Some of the selected polymers exhibit alithium-ion conductivity greater than 10⁻² S/cm. In some embodiments,the high-elasticity polymer is a neat polymer containing no additive orfiller dispersed therein. In others, the high-elasticity polymer is apolymer matrix composite containing from 0.1% to 50% by weight(preferably from 1% to 35% by weight) of a lithium ion-conductingadditive dispersed in a polymer matrix material. In some embodiments,the high-elasticity polymer contains from 0.1% by weight to 10% byweight of a reinforcement nanofilament selected from carbon nanotube,carbon nanofiber, graphene, or a combination thereof.

In some embodiments, the high-elasticity polymer is mixed with anelastomer (to form a blend, co-polymer, or interpenetrating network)selected from natural polyisoprene (e.g. cis-1,4-polyisoprene naturalrubber (NR) and trans-1,4-polyisoprene gutta-percha), syntheticpolyisoprene (IR for isoprene rubber), polybutadiene (BR for butadienerubber), chloroprene rubber (CR), polychloroprene (e.g. Neoprene,Baypren etc.), butyl rubber (copolymer of isobutylene and isoprene,IIR), including halogenated butyl rubbers (chloro butyl rubber (CIIR)and bromo butyl rubber (BIIR), styrene-butadiene rubber (copolymer ofstyrene and butadiene, SBR), nitrile rubber (copolymer of butadiene andacrylonitrile, NBR), EPM (ethylene propylene rubber, a copolymer ofethylene and propylene), EPDM rubber (ethylene propylene diene rubber, aterpolymer of ethylene, propylene and a diene-component),epichlorohydrin rubber (ECO), polyacrylic rubber (ACM, ABR), siliconerubber (SI, Q, VMQ), fluorosilicone rubber (FVMQ), fluoroelastomers(FKM, and FEPM; such as Viton, Tecnoflon, Fluorel, Aflas and Dai-El),perfluoroelastomers (FFKM: Tecnoflon PFR, Kalrez, Chemraz, Perlast),polyether block amides (PEBA), chlorosulfonated polyethylene (CSM; e.g.Hypalon), and ethylene-vinyl acetate (EVA), thermoplastic elastomers(TPE), protein resilin, protein elastin, ethylene oxide-epichlorohydrincopolymer, polyurethane, urethane-urea copolymer, and combinationsthereof.

In some embodiments, the high-elasticity polymer further contains alithium ion-conducting additive dispersed in a high-elasticity polymermatrix material, wherein the lithium ion-conducting additive is selectedfrom Li₂CO₃, Li₂O, Li₂C₂O₄, LiOH, LiX, ROCO₂Li, HCOLi, ROLi, (ROCO₂Li)₂,(CH₂OCO₂Li)₂, Li₂S, Li_(x)SO_(y), or a combination thereof, wherein X═F,Cl, I, or Br, R=a hydrocarbon group, 0<x≤1 and 1≤y≤4.

The high-elasticity polymer may form a mixture, blend, co-polymer, orsemi-interpenetrating network (semi-IPN) with an electron-conductingpolymer selected from polyaniline, polypyrrole, polythiophene,polyfuran, a bi-cyclic polymer, derivatives thereof (e.g. sulfonatedversions), or a combination thereof.

In some embodiments, the high-elasticity polymer may form a mixture,blend, or semi-IPN with a lithium ion-conducting polymer selected frompoly(ethylene oxide) (PEO), Polypropylene oxide (PPO),poly(acrylonitrile) (PAN), poly(methyl methacrylate) (PMMA),poly(vinylidene fluoride) (PVDF), Poly bis-methoxyethoxyethoxide-phosphazenex, Polyvinyl chloride, Polydimethylsiloxane,poly(vinylidene fluoride)-hexafluoropropylene (PVDF-HFP), a sulfonatedderivative thereof, or a combination thereof. Sulfonation is hereinfound to impart improved lithium ion conductivity to a polymer.

The cathode active material may be selected from an inorganic material,an organic material, a polymeric material, or a combination thereof. Theinorganic material may be selected from a metal oxide, metal phosphate,metal silicide, metal selenide, metal sulfide, or a combination thereof.

The inorganic material may be selected from a lithium cobalt oxide,lithium nickel oxide, lithium manganese oxide, lithium vanadium oxide,lithium-mixed metal oxide, lithium iron phosphate, lithium manganesephosphate, lithium vanadium phosphate, lithium mixed metal phosphate,lithium metal silicide, or a combination thereof.

In certain preferred embodiments, the inorganic material is selectedfrom a metal fluoride or metal chloride including the group consistingof CoF₃, MnF₃, FeF₃, VF₃, VOF₃, TiF₃, BiF₃, NiF₂, FeF₂, CuF₂, CuF, SnF₂,AgF, CuCl₂, FeCl₃, MnCl₂, and combinations thereof. In certain preferredembodiments, the inorganic material is selected from a lithiumtransition metal silicate, denoted as Li₂MSiO₄ or Li₂Ma_(x)Mb_(y)SiO₄,wherein M and Ma are selected from Fe, Mn, Co, Ni, V, or VO; Mb isselected from Fe, Mn, Co, Ni, V, Ti, Al, B, Sn, or Bi; and x+y≤1.

In certain preferred embodiments, the inorganic material is selectedfrom a transition metal dichalcogenide, a transition metaltrichalcogenide, or a combination thereof. The inorganic material isselected from TiS₂, TaS₂, MoS₂, NbSe₃, MnO₂, CoO₂, an iron oxide, avanadium oxide, or a combination thereof.

The cathode active material layer may contain a metal oxide containingvanadium oxide selected from the group consisting of VO₂, Li_(x)VO₂,V₂O₅, Li_(x)V₂O₅, V₃O₈, Li_(x)V₃O₈, Li_(x)V₃O₇, V₄O₉, Li_(x)V₄O₉, V₆O₁₃,Li_(x)V₆O₁₃, their doped versions, their derivatives, and combinationsthereof, wherein 0.1<x<5.

The cathode active material layer may contain a metal oxide or metalphosphate, selected from a layered compound LiMO₂, spinel compoundLiM₂O₄, olivine compound LiMPO₄, silicate compound Li₂MSiO₄, Tavoritecompound LiMPO₄F, borate compound LiMBO₃, or a combination thereof,wherein M is a transition metal or a mixture of multiple transitionmetals.

In some embodiments, the inorganic material is selected from: (a)bismuth selenide or bismuth telluride, (b) transition metaldichalcogenide or trichalcogenide, (c) sulfide, selenide, or tellurideof niobium, zirconium, molybdenum, hafnium, tantalum, tungsten,titanium, cobalt, manganese, iron, nickel, or a transition metal; (d)boron nitride, or (e) a combination thereof.

For a lithium-sulfur cell, the cathode may contain sulfur, asulfur-containing molecule, a sulfur compound, a lithium polysulfide, asulfur/carbon hybrid or composite, a sulfur/graphite hybrid orcomposite, a sulfur/graphene hybrid or composite, a sulfur-polymercompound, or a combination thereof. For a lithium-selenium battery, thecathode contains selenium (Se) or a Se-containing compound as a cathodeactive material.

The cathode active material layer may contain an organic material orpolymeric material selected from Poly(anthraquinonyl sulfide) (PAQS), alithium oxocarbon, 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA),poly(anthraquinonyl sulfide), pyrene-4,5,9,10-tetraone (PYT),polymer-bound PYT, Quino(triazene), redox-active organic material,Tetracyanoquinodimethane (TCNQ), tetracyanoethylene (TCNE),2,3,6,7,10,11-hexamethoxytriphenylene (HMTP), poly(5-amino-1,4-dyhydroxyanthraquinone) (PADAQ), phosphazene disulfide polymer ([(NPS₂)₃]n),lithiated 1,4,5,8-naphthalenetetraol formaldehyde polymer,Hexaazatrinaphtylene (HATN), Hexaazatriphenylene hexacarbonitrile(HAT(CN)₆), 5-Benzylidene hydantoin, Isatine lithium salt, Pyromelliticdiimide lithium salt, tetrahydroxy-p-benzoquinone derivatives (THQLi₄),N,N′-diphenyl-2,3,5,6-tetraketopiperazine (PHP),N,N′-diallyl-2,3,5,6-tetraketopiperazine (AP),N,N′-dipropyl-2,3,5,6-tetraketopiperazine (PRP), a thioether polymer, aquinone compound, 1,4-benzoquinone, 5,7,12,14-pentacenetetrone (PT),5-amino-2,3-dihydro-1,4-dyhydroxy anthraquinone (ADDAQ),5-amino-1,4-dyhydroxy anthraquinone (ADAQ), calixquinone, Li₄C₆O₆,Li₂C₆O₆, Li₆C₆O₆, or a combination thereof.

The thioether polymer is selected fromPoly[methanetetryl-tetra(thiomethylene)] (PMTTM),Poly(2,4-dithiopentanylene) (PDTP), a polymer containingPoly(ethene-1,1,2,2-tetrathiol) (PETT) as a main-chain thioetherpolymers, a side-chain thioether polymer having a main-chain consistingof conjugating aromatic moieties, and having a thioether side chain as apendant, Poly(2-phenyl-1,3-dithiolane) (PPDT),Poly(1,4-di(1,3-dithiolan-2-yl)benzene) (PDDTB),poly(tetrahydrobenzodithiophene) (PTHBDT),poly[1,2,4,5-tetrakis(propylthio)benzene] (PTKPTB, orpoly[3,4(ethylenedithio)thiophene] (PEDTT).

In other embodiments, the cathode active material layer contains anorganic material selected from a phthalocyanine compound, such as copperphthalocyanine, zinc phthalocyanine, tin phthalocyanine, ironphthalocyanine, lead phthalocyanine, nickel phthalocyanine, vanadylphthalocyanine, fluorochromium phthalocyanine, magnesium phthalocyanine,manganous phthalocyanine, dilithium phthalocyanine, aluminumphthalocyanine chloride, cadmium phthalocyanine, chlorogalliumphthalocyanine, cobalt phthalocyanine, silver phthalocyanine, ametal-free phthalocyanine, a chemical derivative thereof, or acombination thereof.

The cathode active material is preferably in a form of nanoparticle(spherical, ellipsoidal, and irregular shape), nanowire, nanofiber,nanotube, nanosheet, nanobelt, nanoribbon, nanodisc, nanoplatelet, ornanohorn having a thickness or diameter less than 100 nm. These shapescan be collectively referred to as “particles” or “nanoparticles” unlessotherwise specified or unless a specific type among the above species isdesired. Further preferably, the cathode active material has a dimensionless than 50 nm, even more preferably less than 20 nm, and mostpreferably less than 10 nm. In some embodiments, one particle or acluster of particles may be coated with or embraced by a layer of carbondisposed between the particle(s) and/or a high-elasticity polymer layer(an encapsulating shell).

The cathode layer may further contain a graphite, graphene, or carbonmaterial mixed with the cathode active material particles. The carbon orgraphite material is selected from polymeric carbon, amorphous carbon,chemical vapor deposition carbon, coal tar pitch, petroleum pitch,mesophase pitch, carbon black, coke, acetylene black, activated carbon,fine expanded graphite particle with a dimension smaller than 100 nm,artificial graphite particle, natural graphite particle, or acombination thereof. Graphene may be selected from pristine graphene,graphene oxide, reduced graphene oxide, graphene fluoride, hydrogenatedgraphene, nitrogenated graphene, functionalized graphene, etc.

The cathode active material particles may be coated with or embraced bya conductive protective coating, selected from a carbon material,graphene, electronically conductive polymer, conductive metal oxide, orconductive metal coating. Preferably, the cathode active material, inthe form of a nanoparticle, nanowire, nanofiber, nanotube, nanosheet,nanobelt, nanoribbon, nanodisc, nanoplatelet, or nanohorn ispre-intercalated or pre-doped with lithium ions to form a prelithiatedanode active material having an amount of lithium from 0.1% to 54.7% byweight of said prelithiated anode active material.

The present invention also provides a lithium metal-air batterycomprising an air cathode, an anode comprising a high-elasticity polymerbased protective layer as defined above, and electrolyte, or electrolytecombined with a separator, disposed between the anode and the aircathode. In the air cathode, oxygen from the open air (or from an oxygensupplier external to the battery) is the primary cathode activematerial. The air cathode needs an inert material to support the lithiumoxide material formed at the cathode. The applicants have surprisinglyfound that an integrated structure of conductive nanofilaments can beused as an air cathode intended for supporting the discharge product(e.g., lithium oxide).

Hence, a further embodiment of the present invention is a lithiummetal-air battery, wherein the air cathode comprises an integratedstructure of electrically conductive nanometer-scaled filaments that areinterconnected to form a porous network of electron-conducting pathscomprising interconnected pores, wherein the filaments have a transversedimension less than 500 nm (preferably less than 100 nm). Thesenanofilaments can be selected from carbon nanotubes (CNTs), carbonnanofibers (CNFs), graphene sheets, carbon fibers, graphite fibers, etc.

The invention also provides a method of manufacturing a lithium battery,the method comprising: (a) providing a cathode active material layer andan optional cathode current collector to support the cathode activematerial layer; (b) providing an anode active material layer (containinga lithium metal or lithium alloy foil or coating) and an optional anodecurrent collector to support the lithium metal or lithium alloy foil orcoating; (c) providing an electrolyte in contact with the anode activematerial layer and the cathode active material layer and an optionalseparator electrically separating the anode and the cathode; and (d)providing an anode-protecting layer of a high-elasticity polymer havinga recoverable tensile elastic strain from 2% to 1,500% (preferably from5% to 1,000%), a lithium ion conductivity no less than 10⁻⁶ S/cm at roomtemperature, and a thickness from 0.5 nm to 10 μm. This anode-protectinglayer is disposed between the lithium metal or lithium alloy foil orcoating and the porous separator.

Preferably, the high-elasticity polymer has a lithium ion conductivityfrom 1×10⁻⁵ S/cm to 5×10⁻² S/cm. In some embodiments, thehigh-elasticity polymer has a recoverable tensile strain from 10% to300% (more preferably >30%, and further more preferably >50%).

In certain embodiments, the procedure of providing a high-elasticitypolymer contains providing a mixture/blend/composite of a rotaxanepolymer with an elastomer, an electronically conductive polymer (e.g.polyaniline, polypyrrole, polythiophene, polyfuran, a bi-cyclic polymer,a sulfonated derivative thereof, or a combination thereof), alithium-ion conducting material, a reinforcement material (e.g. carbonnanotube, carbon nanofiber, and/or graphene), or a combination thereof.

In this mixture/blend/composite, the lithium ion-conducting material isdispersed in the high-elasticity polymer and is preferably selected fromLi₂CO₃, Li₂O, Li₂C₂O₄, LiOH, LiX, ROCO₂Li, HCOLi, ROLi, (ROCO₂Li)₂,(CH₂OCO₂Li)₂, Li₂S, Li_(x)SO_(y), or a combination thereof, wherein X═F,Cl, I, or Br, R=a hydrocarbon group, 0<x<1 and 1<y<4.

In some embodiments, the lithium ion-conducting material is dispersed inthe high-elasticity polymer and is selected from lithium perchlorate,LiClO₄, lithium hexafluoro-phosphate, LiPF₆, lithium borofluoride,LiBF₄, lithium hexafluoroarsenide, LiAsF₆, lithiumtrifluoro-metasulfonate, LiCF₃SO₃, bis-trifluoromethyl sulfonylimidelithium, LiN(CF₃SO₂)₂, lithium bis(oxalato)borate, LiBOB, lithiumoxalyldifluoroborate, LiBF₂C₂O₄, lithium oxalyldifluoroborate,LiBF₂C₂O₄, lithium nitrate, LiNO₃, Li-Fluoroalkyl-Phosphates,LiPF₃(CF₂CF₃)₃, lithium bisperfluoro-ethysulfonylimide, LiBETI, lithiumbis(trifluoromethanesulphonyl)imide, lithium bis(fluorosulphonyl)imide,lithium trifluoromethanesulfonimide, LiTFSI, an ionic liquid-basedlithium salt, or a combination thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 Schematic of a prior art lithium metal battery cell, containingan anode layer (a thin Li foil or Li coating deposited on a surface of acurrent collector, Cu foil), a porous separator, and a cathode activematerial layer, which is composed of particles of a cathode activematerial, a conductive additive (not shown) and a resin binder (notshown). A cathode current collector supporting the cathode active layeris also shown.

FIG. 2 Schematic of a presently invented lithium metal battery cellcontaining an anode layer (a thin Li foil or Li coating deposited on asurface of a current collector, Cu foil), a high-elasticitypolymer-based anode-protecting layer, a porous separator, and a cathodeactive material layer, which is composed of particles of a cathodeactive material, a conductive additive (not shown) and a resin binder(not shown). A cathode current collector supporting the cathode activelayer is also shown.

FIG. 3(A) Representative tensile stress-strain curves of some rotaxanenetwork polymers.

FIG. 3(B) The specific intercalation capacity curves of four lithiumcells: 2 cells each having a cathode containing V₂O₅ particles (one cellhaving a rotaxane polymer-based protective layer and the other not) and2 cells each having a cathode containing graphene-embraced V₂O₅particles (one cell having a rotaxane polymer-based protective layer andthe other not).

FIG. 4(A) Representative tensile stress-strain curves of polyrotaxanenetwork films.

FIG. 4(B) The specific capacity values of two lithium-LiCoO₂ cells(initially the cell being lithium-free) featuring (1) high-elasticitypolyrotaxane network layer at the anode and (2) no polymer protectionlayer at the anode, respectively.

FIG. 5 The discharge capacity curves of two coin cells having aFeF₃-based of cathode active materials: (1) having a high-elasticitypolyrotaxane network-protected anode; and (2) no anode-protecting layer.

FIG. 6 Specific capacities of two lithium-FePc (organic) cells, eachhaving Li foil as an anode active material and FePc/RGO mixtureparticles as the cathode active material (one cell containing apolyrotaxane network-protected anode and the other no anode protectionlayer).

FIG. 7 The cathode specific capacity values of two Li—S battery having acathode active material based on a S-impregnated activated MCMBparticles: one cell having a polyrotaxane network-protected anode andthe other having no anode protection layer.

FIG. 8 The cathode specific capacity values of two Li—S batteries havinga S/graphene hybrid-based cathode active material and (1) ahigh-elasticity polyrotaxane network layer for anode protection and (2)no anode protection layer, respectively.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

This invention is directed at a lithium secondary battery, which ispreferably based on an organic electrolyte, a polymer gel electrolyte,an ionic liquid electrolyte, a quasi-solid electrolyte, or a solid-stateelectrolyte. The shape of a lithium secondary battery can becylindrical, square, button-like, etc. The present invention is notlimited to any battery shape or configuration or any type ofelectrolyte.

The invention provides a lithium secondary battery, comprising acathode, an anode, and electrolyte or separator-electrolyte assemblydisposed between the cathode and the anode, wherein the anode comprises:(a) a foil or coating of lithium or lithium alloy as an anode activematerial; and (b) a thin layer of a high-elasticity polymer having arecoverable tensile strain no less than 2%, a lithium ion conductivityno less than 10⁻⁶ S/cm at room temperature, and a thickness from 1 nm to10 μm, wherein the high-elasticity polymer is disposed (interposed)between the lithium or lithium alloy foil/coating and the porousseparator. The foil or coating of lithium or lithium alloy may besupported by a current collector (e.g. a Cu foil, a Ni foam, a porouslayer of nanofilaments, such as graphene sheets, carbon nanofibers,carbon nanotubes, etc. forming a 3D interconnected network ofelectron-conducting pathways).

Preferably, this anode-protecting layer is different in composition thanthe electrolyte used in the lithium battery and maintains as a discretelayer (not to be dissolved in the electrolyte) that is disposed betweenthe anode active material layer (e.g. Li foil or Li coating on a currentcollector) and the electrolyte (or electrolyte-separator layer).

We have discovered that this protective layer provides severalunexpected benefits: (a) the formation of dendrite has been essentiallyeliminated; (b) uniform deposition of lithium back to the anode side isreadily achieved; (c) the layer ensures smooth and uninterruptedtransport of lithium ions from/to the lithium foil/coating and throughthe interface between the lithium foil/coating and the protective layerwith minimal interfacial resistance; and (d) cycle stability can besignificantly improved and cycle life increased.

In a conventional lithium metal cell, as illustrated in FIG. 1, theanode active material (lithium) is deposited in a thin film form or athin foil form directly onto an anode current collector (e.g. a Cufoil). The battery is a lithium metal battery, lithium sulfur battery,lithium-air battery, lithium-selenium battery, etc. As previouslydiscussed in the Background section, these lithium secondary batterieshave the dendrite-induced internal shorting and “dead lithium” issues atthe anode.

We have solved these challenging issues that have troubled batterydesigners and electrochemists alike for more than 30 years by developingand implementing a new anode-protecting layer disposed between thelithium foil/coating and the separator layer. This protective layercomprises a high-elasticity polymer having a recoverable (elastic)tensile strain no less than 2% (preferably no less than 5%) underuniaxial tension and a lithium ion conductivity no less than 10⁻⁶ S/cmat room temperature (preferably and more typically from 1×10⁻⁵ S/cm to5×10⁻² S/cm). The high-elasticity polymer contains a polyrotaxanenetwork having a rotaxane structure or a polyrotaxane structure at acrosslink point of said polyrotaxane network.

As schematically shown in FIG. 2, one embodiment of the presentinvention is a lithium metal battery cell containing an anode layer (athin Li foil or Li coating deposited on a surface of a currentcollector, Cu foil), a high-elasticity polymer-based anode-protectinglayer, a porous separator, and a cathode active material layer, which iscomposed of particles of a cathode active material, a conductiveadditive (not shown) and a resin binder (not shown). A cathode currentcollector (e.g. Al foil) supporting the cathode active layer is alsoshown in FIG. 2.

High-elasticity polymer refers to a polymer that exhibits an elasticdeformation of at least 2% when measured under uniaxial tension. In thefield of materials science and engineering, the “elastic deformation” isdefined as a deformation of a material (when being mechanicallystressed) that is essentially fully recoverable and the recovery isessentially instantaneous upon release of the load. The elasticdeformation is preferably greater than 5%, more preferably greater than10%, further more preferably greater than 30%, and still more preferablygreater than 100%.

It may be noted that although FIG. 2 shows a lithium coatingpre-existing at the anode when the lithium battery is made, this is butone embodiment of the instant invention. An alternative embodiment is alithium battery that does not contain a lithium foil or lithium coatingat the anode (only an anode current collector, such as a Cu foil or agraphene/CNT mat) when the battery is made. The needed lithium to bebounced back and forth between the anode and the cathode is initiallystored in the cathode active material (e.g. lithium vanadium oxideLi_(x)V₂O₅, instead of vanadium oxide, V₂O₅; or lithium polysulfide,instead of sulfur). During the first charging procedure of the lithiumbattery (e.g. as part of the electrochemical formation process), lithiumcomes out of the cathode active material, migrates to the anode side,and deposits on the anode current collector. The presence of thepresently invented high-elasticity polymer layer enables the uniformdeposition of lithium ions on the anode current collector surface. Suchan alternative battery configuration avoids the need to have a layer oflithium foil or coating being present during battery fabrication. Barelithium metal is highly sensitive to air moisture and oxygen and, thus,is more challenging to handle in a real battery manufacturingenvironment. This strategy of pre-storing lithium in the lithiated(lithium-containing) cathode active materials, such as Li_(x)V₂O₅ andLi₂S_(x), makes all the materials safe to handle in a real manufacturingenvironment. Cathode active materials, such as Li_(x)V₂O₅ and Li₂S_(x),are typically not air-sensitive.

The presently invented lithium secondary batteries can contain a widevariety of cathode active materials. The cathode active material layermay contain a cathode active material selected from an inorganicmaterial, an organic material, a polymeric material, or a combinationthereof. The inorganic material may be selected from a metal oxide,metal phosphate, metal silicide, metal selenide, transition metalsulfide, or a combination thereof.

The inorganic material may be selected from a lithium cobalt oxide,lithium nickel oxide, lithium manganese oxide, lithium vanadium oxide,lithium-mixed metal oxide, lithium iron phosphate, lithium manganesephosphate, lithium vanadium phosphate, lithium mixed metal phosphate,lithium metal silicide, or a combination thereof.

In certain preferred embodiments, the inorganic material is selectedfrom a metal fluoride or metal chloride including the group consistingof CoF₃, MnF₃, FeF₃, VF₃, VOF₃, TiF₃, BiF₃, NiF₂, FeF₂, CuF₂, CuF, SnF₂,AgF, CuCl₂, FeCl₃, MnCl₂, and combinations thereof. In certain preferredembodiments, the inorganic material is selected from a lithiumtransition metal silicate, denoted as Li₂MSiO₄ or Li₂Ma_(x)Mb_(y)SiO₄,wherein M and Ma are selected from Fe, Mn, Co, Ni, V, or VO; Mb isselected from Fe, Mn, Co, Ni, V, Ti, Al, B, Sn, or Bi; and x+y<1.

In certain preferred embodiments, the inorganic material is selectedfrom a transition metal dichalcogenide, a transition metaltrichalcogenide, or a combination thereof. The inorganic material isselected from TiS₂, TaS₂, MoS₂, NbSe₃, MnO₂, CoO₂, an iron oxide, avanadium oxide, or a combination thereof.

The cathode active material layer may contain a metal oxide containingvanadium oxide selected from the group consisting of VO₂, Li_(x)VO₂,V₂O₅, Li_(x)V₂O₅, V₃O₈, Li_(x)V₃O₈, Li_(x)V₃O₇, V₄O₉, Li_(x)V₄O₉, V₆O₁₃,Li₈V₆O₁₃, their doped versions, their derivatives, and combinationsthereof, wherein 0.1<x<5.

The cathode active material layer may contain a metal oxide or metalphosphate, selected from a layered compound LiMO₂, spinel compoundLiM₂O₄, olivine compound LiMPO₄, silicate compound Li₂MSiO₄, Tavoritecompound LiMPO₄F, borate compound LiMBO₃, or a combination thereof,wherein M is a transition metal or a mixture of multiple transitionmetals.

In some embodiments, the inorganic material is selected from: (a)bismuth selenide or bismuth telluride, (b) transition metaldichalcogenide or trichalcogenide, (c) sulfide, selenide, or tellurideof niobium, zirconium, molybdenum, hafnium, tantalum, tungsten,titanium, cobalt, manganese, iron, nickel, or a transition metal; (d)boron nitride, or (e) a combination thereof.

The cathode active material layer may contain an organic material orpolymeric material selected from Poly(anthraquinonyl sulfide) (PAQS), alithium oxocarbon, 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA),poly(anthraquinonyl sulfide), pyrene-4,5,9,10-tetraone (PYT),polymer-bound PYT, Quino(triazene), redox-active organic material,Tetracyanoquinodimethane (TCNQ), tetracyanoethylene (TCNE),2,3,6,7,10,11-hexamethoxytriphenylene (HMTP), poly(5-amino-1,4-dyhydroxyanthraquinone) (PADAQ), phosphazene disulfide polymer ([(NPS₂)₃]n),lithiated 1,4,5,8-naphthalenetetraol formaldehyde polymer,Hexaazatrinaphtylene (HATN), Hexaazatriphenylene hexacarbonitrile(HAT(CN)₆), 5-Benzylidene hydantoin, Isatine lithium salt, Pyromelliticdiimide lithium salt, tetrahydroxy-p-benzoquinone derivatives (THQLi₄),N,N′-diphenyl-2,3,5,6-tetraketopiperazine (PHP),N,N′-diallyl-2,3,5,6-tetraketopiperazine (AP),N,N′-dipropyl-2,3,5,6-tetraketopiperazine (PRP), a thioether polymer, aquinone compound, 1,4-benzoquinone, 5,7,12,14-pentacenetetrone (PT),5-amino-2,3-dihydro-1,4-dyhydroxy anthraquinone (ADDAQ),5-amino-1,4-dyhydroxy anthraquinone (ADAQ), calixquinone, Li₄C₆O₆,Li₂C₆O₆, Li₆C₆O₆, or a combination thereof.

The thioether polymer is selected fromPoly[methanetetryl-tetra(thiomethylene)] (PMTTM),Poly(2,4-dithiopentanylene) (PDTP), a polymer containingPoly(ethene-1,1,2,2-tetrathiol) (PETT) as a main-chain thioetherpolymers, a side-chain thioether polymer having a main-chain consistingof conjugating aromatic moieties, and having a thioether side chain as apendant, Poly(2-phenyl-1,3-dithiolane) (PPDT),Poly(1,4-di(1,3-dithiolan-2-yl)benzene) (PDDTB),poly(tetrahydrobenzodithiophene) (PTHBDT),poly[1,2,4,5-tetrakis(propylthio)benzene] (PTKPTB, orpoly[3,4(ethylenedithio)thiophene] (PEDTT).

In other embodiments, the cathode active material layer contains anorganic material selected from a phthalocyanine compound, such as copperphthalocyanine, zinc phthalocyanine, tin phthalocyanine, ironphthalocyanine, lead phthalocyanine, nickel phthalocyanine, vanadylphthalocyanine, fluorochromium phthalocyanine, magnesium phthalocyanine,manganous phthalocyanine, dilithium phthalocyanine, aluminumphthalocyanine chloride, cadmium phthalocyanine, chlorogalliumphthalocyanine, cobalt phthalocyanine, silver phthalocyanine, ametal-free phthalocyanine, a chemical derivative thereof, or acombination thereof.

The lithium secondary battery may be a lithium-sulfur battery, whereinthe cathode comprises sulfur, a sulfur-containing molecule, asulfur-containing compound, a metal sulfide, a sulfur-carbon polymer, alithium polysulfide, a sulfur/carbon hybrid or composite, asulfur/graphite hybrid or composite, a sulfur/graphene hybrid orcomposite, a sulfur-polymer compound, or a combination thereof

In the rechargeable lithium-sulfur cell, the metal sulfide may contain amaterial denoted by M_(x)S_(y), wherein x is an integer from 1 to 3 andy is an integer from 1 to 10, and M is a metal element selected from analkali metal, an alkaline metal selected from Mg or Ca, a transitionmetal, a metal from groups 13 to 17 of the periodic table, or acombination thereof. The metal element M preferably is selected from Li,Na, K, Mg, Zn, Cu, Ti, Ni, Co, Fe, or Al. In some preferred embodiments,the metal sulfide in the cathode layer contains Li₂S₁, Li₂S₂, Li₂S₃,Li₂S₄, Li₂S₅, Li₂S₆, Li₂S₇, Li₂S₈, Li₂S₉, Li₂S₁₀, or a combinationthereof.

Preferably and typically, the high-elasticity polymer has a lithium ionconductivity no less than 10⁻⁵ S/cm, more preferably no less than 10⁻⁴S/cm, further preferably no less than 10⁻³ S/cm, and most preferably noless than 10⁻² S/cm. In some embodiments, the high-elasticity polymer isa neat polymer having no additive or filler dispersed therein. Inothers, the high-elasticity polymer is a polymer matrix compositecontaining from 0.1% to 50% (preferably 1% to 35%) by weight of alithium ion-conducting additive dispersed in a high-elasticity polymermatrix material. The high-elasticity polymer must have a high elasticity(elastic deformation strain value >2%). An elastic deformation is adeformation that is fully recoverable and the recovery process isessentially instantaneous (no significant time delay). Thehigh-elasticity polymer can exhibit an elastic deformation from 5% up to1,500% (15 times of its original length), more typically from 10% to800%, and further more typically from 50% to 500%, and most typicallyand desirably from 70% to 300%. It may be noted that although a metaltypically has a high ductility (i.e. can be extended to a large extentwithout breakage), the majority of the deformation is plasticdeformation (non-recoverable) and only a small amount of elasticdeformation (typically <1% and more typically <0.2%).

In some preferred embodiments, the high-elasticity polymer contains apolyrotaxane network having a rotaxane structure or a polyrotaxanestructure at a crosslink point of said polyrotaxane network. Thesenetwork or cross-linked polymers exhibit a unique combination of a highelasticity (high elastic deformation strain) and high lithium-ionconductivity.

The rotaxane structure or polyrotaxane structure may be selected fromrotaxane, a chemically modified rotaxane (rotaxane derivative), apolymer-grafted rotaxane, polyrotaxane, a co-polymer of polyrotaxane, agraft polymer of polyrotaxane, a polymer blend of polymer ofpolyrotaxane, a chemically modified polyrotaxane, or a combinationthereof. These network or cross-linked polymers exhibit a uniquecombination of a high elasticity (high elastic deformation strain) andhigh lithium-ion conductivity.

In certain embodiments, the polyrotaxane network contains a polymerselected from polyethylene glycol, polypropylene glycol, polyethyleneoxide, polypropylene oxide, poly (succinic acid), an aliphaticpolyester, or a combination thereof. These network or cross-linkedpolymers exhibit a unique combination of a high elasticity (high elasticdeformation strain) and high lithium-ion conductivity.

A polyrotaxane network is a network polymer having a rotaxane orpolyrotaxane structure at the crosslink points. A polyrotaxane typicallycontains many cyclic molecules that are threaded on a single polymerChain, which is trapped by capping the chain with bulky end groups. Oneexample is a polyrotaxane consisting of α-cyclodextrin (α-CD) andpoly(ethylene glycol) (PEG), wherein a PEG chain penetrates multipleα-CD rings, illustrated below in Schematic A below as an example.

It is also possible to have multiple α-CD rings being cross-linkedtogether (e.g. by cyanuric chloride) to form a 3D network of chains. Inthis network, the polymer chains with bulky end groups (e.g. bisamine)are neither covalently cross-linked nor do they form conventionalphysical entanglements. Instead, they are topologically interlocked by“figure-of-eight” cross-links, as illustrated in Schematic B below.These cross-links can pass along the polymer chains freely to relax outthe stress exerted on the threaded polymer chains just like pulleys.This topological network by figure-of-eight cross-links is hereinreferred to as a polyrotaxane (PR) network.

Furthermore, the α-CDs in the PR may be modified with polymerizablemolecules (e.g. vinyl molecules), so that the PR derivative becomes across-linker for preparing complex 3D polymer networks, such as polymergels. Molecules of α-CDs may also be modified with othermulti-functionality molecules, such as —COOH and —OOCHN—R (R=methyl orother alkyl groups). The cyclodextrin (CD) may be a permethylated CD.The α-CDs in the PR may also be grafted with a polymer. For instance,poly(N-isopropyl acrylamide) (PNIPA) may be grafted from α-cyclodextrinof PR, via controlled radical polymerization. The terminal chlorinatedalkyl group of the grafted PNIPA may then be modified with azide oralkyne. As a result, one obtains several types of PNIPA-grafted PRmolecules with different terminations of PNIPA as building blocks toprepare 3D crosslinked network polymers having a high elasticity.

A simple and effective protocol has been developed to directly introducerotaxane cross-links into vinyl polymers with a cross-linker, throughthe radical polymerization of the corresponding vinyl monomers [T. Arai,et al. “Versatile supramolecular cross-linker: a rotaxane cross-linkerthat directly endows vinyl polymers with movable cross-links,” Chemistry19, 5917-5923 (2013)]. This protocol for a rotaxane-crosslinked polymer(RCP) is achieved by transforming the cross-link structure of thecross-linker without requiring the pre-synthesis or cross-linking ofpolyrotaxane. The crosslinker is a CD-based vinylic supramolecularcross-linker (VSC) capable of facilitating the synthesis of polyrotaxanenetworks through radical polymerization of a vinyl monomer.

To prepare the VSC, an oligomacrocycle and a macromonomer with a bulkyend-group are mixed to form a cross-linked inclusion complex throughpseudo-rotaxanation. Successive radical polymerization of the vinylmonomer in the presence of VSC yields RCP possessing movable cross-linksor movable polymer chains at the cross-link points. In this system, thepseudo-polyrotaxane network structure of the VSC is fixed into thepolymer through copolymerization with the vinyl monomer.

Further, we have unexpectedly discovered that the presence of an amountof a lithium salt or sodium salt (1-35% by weight) and a liquid solvent(0-50%) can significantly increase the lithium-ion or sodium ionconductivity.

Typically, a high-elasticity polymer is originally in a monomer oroligomer states that can be cured to form a cross-linked polymer that ishighly elastic. Prior to curing, these polymers or oligomers are solublein an organic solvent to form a polymer solution. An ion-conducting orelectron-conducting additive may be added to this solution to form asuspension. This solution or suspension can then be formed into a thinlayer of polymer precursor on a surface of an anode current collector.The polymer precursor (monomer or oligomer and initiator) is thenpolymerized and cured to form a lightly cross-linked polymer. This thinlayer of polymer may be tentatively deposited on a solid substrate (e.g.surface of a polymer or glass), dried, and separated from the substrateto become a free-standing polymer layer. This free-standing layer isthen laid on a lithium foil/coating or implemented between a lithiumfilm/coating and electrolyte or separator. Polymer layer formation canbe accomplished by using one of several procedures well-known in theart; e.g. spraying, spray-painting, printing, coating, extrusion-basedfilm-forming, casting, etc.

It is preferred that these materials form a lightly cross-linked networkof polymer chains. In other words, the network polymer or cross-linkedpolymer should have a relatively low degree of cross-linking or lowcross-link density to impart a high elastic deformation. The cross-linkdensity of a cross-linked network of polymer chains may be defined asthe inverse of the molecular weight between cross-links (Mc). Thecross-link density can be determined by the equation, Mc=ρRT/Ge, whereGe is the equilibrium modulus as determined by a temperature sweep indynamic mechanical analysis, ρ is the physical density, R is theuniversal gas constant in J/mol*K and T is absolute temperature in K.Once Ge and ρ are determined experimentally, then Mc and the cross-linkdensity can be calculated.

The magnitude of Mc may be normalized by dividing the Mc value by themolecular weight of the characteristic repeat unit in the cross-linkchain or chain linkage to obtain a number, Nc, which is the number ofrepeating units between two cross-link points. We have found that theelastic deformation strain correlates very well with Mc and Nc. Theelasticity of a cross-linked polymer derives from a large number ofrepeating units (large Nc) between cross-links. The repeating units canassume a more relax conformation (e.g. random coil) when the polymer isnot stressed. However, when the polymer is mechanically stressed, thelinkage chain uncoils or gets stretched to provide a large deformation.A long chain linkage between cross-link points (larger Nc) enables alarger elastic deformation. Upon release of the load, the linkage chainreturns to the more relaxed or coiled state. During mechanical loadingof a polymer, the cross-links prevent slippage of chains that otherwiseform plastic deformation (non-recoverable).

Preferably, the Nc value in a high-elasticity polymer is greater than 5,more preferably greater than 10, further more preferably greater than100, and even more preferably greater than 200. These Nc values can bereadily controlled and varied to achieve different elastic deformationvalues by using different cross-linking agents with differentfunctionalities, and by designing the polymerization and cross-linkingreactions to proceed at different temperatures for different periods oftime.

Alternatively, Mooney-Rilvin method may be used to determine the degreeof cross-linking. Crosslinking also can be measured by swellingexperiments. In a swelling experiment, the crosslinked sample is placedinto a good solvent for the corresponding linear polymer at a specifictemperature, and either the change in mass or the change in volume ismeasured. The higher the degree of crosslinking, the less swelling isattainable. Based on the degree of swelling, the Flory InteractionParameter (which relates the solvent interaction with the sample, FloryHuggins Eq.), and the density of the solvent, the theoretical degree ofcrosslinking can be calculated according to Flory's Network Theory. TheFlory-Rehner Equation can be useful in the determination ofcross-linking.

The aforementioned high-elasticity polymers may be used alone to protectthe lithium foil/coating layer at the anode. Alternatively, thehigh-elasticity polymer can be mixed with a broad array of elastomers,electrically conducting polymers, lithium ion-conducting materials,and/or strengthening materials (e.g. carbon nanotube, carbon nanofiber,or graphene sheets).

A broad array of elastomers can be mixed with a high-elasticity polymerto form a blend, co-polymer, or interpenetrating network thatencapsulates the cathode active material particles. The elastomericmaterial may be selected from natural polyisoprene (e.g.cis-1,4-polyisoprene natural rubber (NR) and trans-1,4-polyisoprenegutta-percha), synthetic polyisoprene (IR for isoprene rubber),polybutadiene (BR for butadiene rubber), chloroprene rubber (CR),polychloroprene (e.g. Neoprene, Baypren etc.), butyl rubber (copolymerof isobutylene and isoprene, IIR), including halogenated butyl rubbers(chloro butyl rubber (CIIR) and bromo butyl rubber (BIIR),styrene-butadiene rubber (copolymer of styrene and butadiene, SBR),nitrile rubber (copolymer of butadiene and acrylonitrile, NBR), EPM(ethylene propylene rubber, a copolymer of ethylene and propylene), EPDMrubber (ethylene propylene diene rubber, a terpolymer of ethylene,propylene and a diene-component), epichlorohydrin rubber (ECO),polyacrylic rubber (ACM, ABR), silicone rubber (SI, Q, VMQ),fluorosilicone rubber (FVMQ), fluoroelastomers (FKM, and FEPM; such asViton, Tecnoflon, Fluorel, Aflas and Dai-El), perfluoroelastomers (FFKM:Tecnoflon PFR, Kalrez, Chemraz, Perlast), polyether block amides (PEBA),chlorosulfonated polyethylene (CSM; e.g. Hypalon), and ethylene-vinylacetate (EVA), thermoplastic elastomers (TPE), protein resilin, proteinelastin, ethylene oxide-epichlorohydrin copolymer, polyurethane,urethane-urea copolymer, and combinations thereof.

In some embodiments, a high-elasticity polymer can form a polymer matrixcomposite containing a lithium ion-conducting additive dispersed in thehigh-elasticity polymer matrix material, wherein the lithiumion-conducting additive is selected from Li₂CO₃, Li₂O, Li₂C₂O₄, LiOH,LiX, ROCO₂Li, HCOLi, ROLi, (ROCO₂Li)₂, (CH₂OCO₂Li)₂, Li₂S, Li_(x)SO_(y),or a combination thereof, wherein X═F, Cl, I, or Br, R=a hydrocarbongroup, 0<x≤1 and 1≤y≤4.

In some embodiments, the high-elasticity polymer can be mixed with alithium ion-conducting additive, which contains a lithium salt selectedfrom lithium perchlorate, LiClO₄, lithium hexafluorophosphate, LiPF₆,lithium borofluoride, LiBF₄, lithium hexafluoroarsenide, LiAsF₆, lithiumtrifluoro-metasulfonate, LiCF₃SO₃, bis-trifluoromethyl sulfonylimidelithium, LiN(CF₃SO₂)₂, lithium bis(oxalato)borate, LiBOB, lithiumoxalyldifluoroborate, LiBF₂C₂O₄, lithium oxalyldifluoroborate,LiBF₂C₂O₄, lithium nitrate, LiNO₃, Li-Fluoroalkyl-Phosphates,LiPF₃(CF₂CF₃)₃, lithium bisperfluoro-ethysulfonylimide, LiBETI, lithiumbis(trifluoromethanesulphonyl)imide, lithium bis(fluorosulphonyl)imide,lithium trifluoromethanesulfonimide, LiTFSI, an ionic liquid-basedlithium salt, or a combination thereof.

The high-elasticity polymer may form a mixture, blend, orinterpenetrating network with an electron-conducting polymer selectedfrom polyaniline, polypyrrole, polythiophene, polyfuran, a bi-cyclicpolymer, derivatives thereof (e.g. sulfonated versions), or acombination thereof. In some embodiments, the high-elasticity polymermay form a mixture, co-polymer, or semi-interpenetrating network with alithium ion-conducting polymer selected from poly(ethylene oxide) (PEO),Polypropylene oxide (PPO), poly(acrylonitrile) (PAN), poly(methylmethacrylate) (PMMA), poly(vinylidene fluoride) (PVDF), Poly bis-methoxyethoxyethoxide-phosphazenex, Polyvinyl chloride, Polydimethylsiloxane,poly(vinylidene fluoride)-hexafluoropropylene (PVDF-HFP), a derivativethereof (e.g. sulfonated versions), or a combination thereof.

Unsaturated rubbers that can be mixed with the high-elasticity polymerinclude natural polyisoprene (e.g. cis-1,4-polyisoprene natural rubber(NR) and trans-1,4-polyisoprene gutta-percha), synthetic polyisoprene(IR for isoprene rubber), polybutadiene (BR for butadiene rubber),chloroprene rubber (CR), polychloroprene (e.g. Neoprene, Baypren etc.),butyl rubber (copolymer of isobutylene and isoprene, IIR), includinghalogenated butyl rubbers (chloro butyl rubber (CIIR) and bromo butylrubber (BIIR), styrene-butadiene rubber (copolymer of styrene andbutadiene, SBR), nitrile rubber (copolymer of butadiene andacrylonitrile, NBR),

Some elastomers are saturated rubbers that cannot be cured by sulfurvulcanization; they are made into a rubbery or elastomeric material viadifferent means: e.g. by having a copolymer domain that holds otherlinear chains together. Each of these elastomers can be used to bondparticles of a cathode active material by one of several means; e.g.spray coating, dilute solution mixing (dissolving the cathode activematerial particles in an uncured polymer, monomer, or oligomer, with orwithout an organic solvent) followed by drying and curing.

Saturated rubbers and related elastomers in this category include EPM(ethylene propylene rubber, a copolymer of ethylene and propylene), EPDMrubber (ethylene propylene diene rubber, a terpolymer of ethylene,propylene and a diene-component), epichlorohydrin rubber (ECO),polyacrylic rubber (ACM, ABR), silicone rubber (SI, Q, VMQ),fluorosilicone rubber (FVMQ), fluoroelastomers (FKM, and FEPM; such asViton, Tecnoflon, Fluorel, Aflas and Dai-El), perfluoroelastomers (FFKM:Tecnoflon PFR, Kalrez, Chemraz, Perlast), polyether block amides (PEBA),chlorosulfonated polyethylene (CSM; e.g. Hypalon), and ethylene-vinylacetate (EVA), thermoplastic elastomers (TPE), protein resilin, andprotein elastin. Polyurethane and its copolymers (e.g. urea-urethanecopolymer) are particularly useful elastomeric shell materials forencapsulating anode active material particles.

Example 1: Lithium Battery Containing a High-ElasticityPolymer-Protected Lithium Anode and a Cathode Containing V₂O₅ Particles

Cathode active material layers were prepared from V₂O₅ particles andgraphene-embraced V₂O₅ particles, respectively. The V₂O₅ particles werecommercially available. Graphene-embraced V₂O₅ particles were preparedin-house. In a typical experiment, vanadium pentoxide gels were obtainedby mixing V₂O₅ in a LiCl aqueous solution. The Lit exchanged gelsobtained by interaction with LiCl solution (the Li:V molar ratio waskept as 1:1) was mixed with a GO suspension and then placed in aTeflon-lined stainless steel 35 ml autoclave, sealed, and heated up to180° C. for 12 h. After such a hydrothermal treatment, the green solidswere collected, thoroughly washed, ultrasonicated for 2 minutes, anddried at 70° C. for 12 h followed by mixing with another 0.1% GO inwater, ultrasonicating to break down nano-belt sizes, and thenspray-drying at 200° C. to obtain graphene-embraced V₂O₅ compositeparticulates. Selected amounts of V₂O₅ particles and graphene-embracedV₂O₅ particles, respectively, were then each made into a cathode layerfollowing a well-known slurry coating process.

Preparation of polyrotaxane film for anode protection was conducted inthe following manner: In an example, polyethylene glycol-bisamine(PEG-BA, 0.9 g) and α-CD (3.6 g) were dissolved in water (30 mL) at 80°C. and kept at 5° C. overnight to yield the white paste of the inclusioncomplex. Then, the paste was dried and added with an excess of2,4-dinitrofluorobenzene (2.4 mL) together with dimethylformamide (10mL) and then the mixture was stirred in a nitrogen atmosphere at roomtemperature overnight. The reaction mixture was dissolved in DMSO (50mL) and precipitated from a 0.1% sodium chloride aqueous solution (800mL) twice to give a yellow product. The product was collected, washedwith water and methanol (three times, respectively), and dried toproduce the polyrotaxane (1.25 g).

The polyrotaxane (100 mg) was dissolved in 1 N NaOH (0.5 mL) at 5° C.The hydroxyl groups of α-CD were ionized under a strong base, whichresulted in Coulombic repulsion between adjacent CDs in thepolyrotaxane. Cyanuric chloride (35 mg), dissolved in 1 N NaOH (0.5 mL),was mixed with the solution to initiate the cross-linking reaction inthe resulting solution. The solution was then quickly cast onto a glasssubstrate to produce a polymer film. The cross-linking reactioncontinued. After 3 h at room temperature, a film of yellow polyrotaxanegel was obtained. This film was peeled off from the glass and cut intopieces with desired dimensions for use as an anode-protecting layer.

Some of the polymer film samples were subjected to dynamic mechanicaltesting to obtain the equilibrium dynamic modulus, Ge, for thedetermination of the number average molecular weight between twocross-link points (Mc) and the corresponding number of repeat units(Nc), as a means of characterizing the degree of cross-linking.

Several tensile testing specimens were cut from each cross-link film andtested with a universal testing machine. The representative tensilestress-strain curves of two polymers are shown in FIG. 3(A), whichindicate that this series of network polymers have an elasticdeformation from approximately 460% (dry network) to 1,490% (swollenwith an organic solvent, EC). These above are for neat polymers withoutany additive. The addition of up to 30% by weight of a lithium salttypically reduces this elasticity down to a reversible tensile strainfrom 10% to 100%.

For electrochemical testing, the working electrodes (cathode layers)were prepared by mixing 85 wt. % V₂O₅ or 88% of graphene-embraced V₂O₅particles, 5-8 wt. % CNTs, and 7 wt. % polyvinylidene fluoride (PVDF)binder dissolved in N-methyl-2-pyrrolidinoe (NMP) to form a slurry of 5wt. % total solid content. After coating the slurries on Al foil, theelectrodes were dried at 120° C. in vacuum for 2 h to remove the solventbefore pressing. Then, the electrodes were cut into a disk (ϕ=12 mm) anddried at 100° C. for 24 h in vacuum.

Electrochemical measurements were carried out using CR2032 (3V)coin-type cells with lithium metal as the counter/reference electrode,Celgard 2400 membrane as separator, and 1 M LiPF₆ electrolyte solutiondissolved in a mixture of ethylene carbonate (EC) and diethyl carbonate(DEC) (EC-DEC, 1:1 v/v). The cell assembly was performed in anargon-filled glove-box. The CV measurements were carried out using aCH-6 electrochemical workstation at a scanning rate of 1 mV/s. Theelectrochemical performance of the cell featuring high-elasticitypolymer binder and that containing PVDF binder were evaluated bygalvanostatic charge/discharge cycling at a current density of 50 mA/gusing an Arbin electrochemical workstation.

Summarized in FIG. 3(B) are the specific intercalation capacity curvesof four lithium cells: 2 cells each having a cathode containing V₂O₅particles (one cell having a polyrotaxane network polymer-based lithiummetal anode-protecting layer and the other not) and 2 cells each havinga cathode containing graphene-embraced V₂O₅ particles (one cell having apolyrotaxane network polymer-based lithium anode-protecting layer andthe other not). As the number of cycles increases, the specific capacityof the un-protected cells drops at the fastest rate. In contrast, thepresently invented polyrotaxane polymer protection layer provides thebattery cell with a significantly more stable and high specific capacityfor a large number of cycles. These data have clearly demonstrated thesurprising and superior performance of the presently inventedpolyrotaxane network polymer protection approach.

The high-elasticity polymer protective layer appears to be capable ofreversibly deforming to a great extent without breakage when the lithiumfoil decreases in thickness during battery discharge. The protectivepolymer layer also prevents the continued reaction between liquidelectrolyte and lithium metal at the anode, reducing the problem ofcontinuing loss in lithium and electrolyte. This also enables asignificantly more uniform deposition of lithium ions upon returningfrom the cathode during a battery re-charge; hence, no lithium dendrite.These were observed by using SEM to examine the surfaces of theelectrodes recovered from the battery cells after some numbers ofcharge-discharge cycles.

Example 2: High-Elasticity Polymer Implemented in the Anode of aLithium-LiCoO₂ Cell (Initially the Cell Anode has an Ultra-Thin LithiumLayer, <1 μm Thick)

The high-elasticity polymer as a lithium-protecting layer was based onanother Rotaxane-based network polymer obtained by following a proceduresimilar to that suggested by Arai, et a. [T. Arai, et al. “Versatilesupramolecular cross-linker: a rotaxane cross-linker that directlyendows vinyl polymers with movable cross-links,” Chemistry 19, 5917-5923(2013)]. First, CD-based vinylic supramolecular cross-linker (VSC) wasprepared by following the procedure described below: An oligomacrocycleand a macromonomer, two constituents of VSCs, were prepared fromcommercially available starting materials. Oligocyclodextrin (OCD) asthe oligomacrocycle was obtained by the controlled reaction ofα-cyclodextrin (α-CD) with a polymer diisocyanate derived frompolypropylene glycol and tolylene diisocyanate (Schematic C).

The average number of α-CD per OCD molecule was 4, as calculated fromthe results of the size-exclusion chromatography (SEC) profile ofacetylated OCD (Mw 8000, polydispersity index (PDI) 1.6), which wasprepared by using acetic anhydride in pyridine. The macromonomer, aterminal bulky end-tethering polyethylene glycol-type methacrylate(TBM), was prepared by the reaction of a hydroxyl-terminated PEG basedmacromonomer with 3,5-dimethylphenyl isocyanate (Schematic D).

OCD and TBM were mixed in alkaline water (0.1M NaOH), and the mixturewas sonicated for 5 min at room temperature to produce a white viscousgel (VSC).

A mixture of N,N-dimethyl-acrylamide (DMAAm, 2.0 g) as a typical vinylmonomer, VSC (0.30 g, 15 wt %), and the photoinitiator (Irgacure-500, 1wt %) in water were cast onto glass surface and UV irradiated at roomtemperature for 3 min to produce a gelled polymer film (84%, RCP-DMAAm).Water was then removed and the film was cut into several pieces. Some ofthe film pieces were impregnated with some organic solvent (e.g. EC). Aseparate polymer composite film (having lithium salt dispersed in thenetwork polymer) was also cast.

Tensile testing was also conducted on the polymer network films (withouthybrid cathode particles) and some testing results are summarized inFIG. 4(A). This series of cross-linked polymers can be elasticallystretched up to approximately 355% (having some lithium salt dispersedtherein) or up to 950% (with no additive).

FIG. 4(B) shows that the cell having an anode-protecting polymer layeroffers a significantly more stable cycling behavior. The high-elasticitypolymer also acts to isolate the electrolyte from the lithium coatingyet still allowing for easy diffusion of lithium ions.

Example 3: Li Metal Cells Containing Metal Fluoride Nanoparticle-BasedCathode and a High-Elasticity Polymer-Protected Lithium Anode

This polyrotaxane (PR) consists of α-cyclodextrin (α-CD), polyethyleneglycol (PEG) with terminal carboxylic acids and a capping agent(1-adamantanamine). In the slide-ring gel, a-CDs in one PR arecross-linked to α-CDs in different PRs. The PEG main chains are notfixed at the cross-linking points in the polymer network; instead, theycan pass through the hole of a figure-8-shaped junction of cross-linkedα-CDs freely, which is called the ‘pulley effect’. The stress exerted onpart of the polymer network is minimized through this effect. As aresult, the polymer network exhibits high extensibility and a smallhysteresis on repeated extension and contraction.

Polymer networks using PR as a cross-linker and N-Isopropylacrylamide(NIPA) as the monomer were prepared. In addition, ionic sites wereintroduced into the PR-cross-linked polymer network to obtain extremelyhigh-elasticity polymer gel. The ionic groups help the PR cross-linkersto become well extended in the polymer network. The resulting polymergels are highly elastic, similar to soft rubbers, because thecross-linked α-CD molecules can move along the PEG chains. A NIPA-basednetwork polymer was herein prepared using a PR modified by2-acryloyloxyethyl isocyanate, which contains both isocyanate and vinylgroups, as the cross-linker (PR—C). The isocyanate groups form stablecarbamate bonds with the α-CD hydroxyl groups in the PR to generate thecross-linking structures.

In an example, hydroxypropylated polyrotaxane HPR (500 mg), a DBTDLcatalyst (1 drop) and BHT (polymerization inhibitor, 0.78 mg) weredissolved in 30 ml of anhydrous DMSO. Then, 2-Acryloyloxyethylisocyanate (78 mg) was dissolved in 10 ml of anhydrous DMSO and thesolution was added dropwise to the mixtures with vigorous stirring inthe absence of light. The mixtures were then continuously stirredovernight at 40° C. to ensure that the reactions were complete. HPR—Cwas re-precipitated from the reaction mixture using an excess ofmethanol or acetone, respectively, and the precipitated product wasrefrigerated. The products were washed several times with methanol andacetone and then dried. The total number of vinyl groups per HPR wasestimated from 1H-NMR spectra to be approximately 200.

The polymer networks were prepared by conventional free-radicalpolymerization of the monomers with the PR cross-linkers. Appropriateamounts of NIPA, AAcNa, HPR—C, and APS (initiator) were dissolved inwater. The final concentrations of the ionic monomer and NIPA were 0.1and 1.9 M, respectively, whereas the crosslinker concentration wasvaried. In all the pre-gel solutions, the total concentration of themonomers, excluding the cross-linkers, was fixed at 2M. Subsequently, N₂gas was bubbled through the pre-gel solutions for 30 min, which werethen sonicated to remove excess nitrogen from the solution. To initiatethe polymerization below room temperature, a few drops of TEMED wereadded to the pre-gel solution and the reacting mass was quickly castonto a glass surface to form several films. The films were polymerizedand cured at 4° C. for 24 h to obtain cross-linked polymer films havingdifferent degrees of cross-linking. Pieces of these films were used tocover (protect) the lithium anode layer of the resulting lithiumbatteries.

In addition, tensile testing was conducted on some cut pieces of thesefilms. This series of cross-linked polymers can be elastically stretchedup to approximately 800% (without any additive). The addition ofadditives results in an elasticity of approximately 5% (20% carbonblack) to 180% (5% graphene sheets, as a conductive additive).

Commercially available powders of CoF₃, MnF₃, FeF₃, VF₃, VOF₃, TiF₃, andBiF₃ were subjected to high-intensity ball-milling to reduce theparticle size down to approximately 0.5-2.3 μm. Each type of these metalfluoride particles, along with graphene sheets (as a conductiveadditive), was then added into an NMP and PVDF binder suspension to forma multiple-component slurry. The slurry was then slurry-coated on Alfoil to form cathode layers.

Shown in FIG. 5 are the discharge capacity curves of two coin cellshaving the same cathode active material (FeF₃), but one cell having ahigh-elasticity polymer-protected anode and the other having noprotective layer. These results have clearly demonstrated that thehigh-elasticity polymer protection strategy provides excellentprotection against capacity decay of a lithium metal battery.

The high-elasticity polymer appears to be capable of reversiblydeforming without breakage when the anode layer expands and shrinksduring charge and discharge. The polymer also prevents continuedreaction between the liquid electrolyte and the lithium metal. Nodendrite-like features were found with the anode being protected by ahigh-elasticity polymer. This was confirmed by using SEM to examine thesurfaces of the electrodes recovered from the battery cells after somenumbers of charge-discharge cycles.

Example 4: Li-Organic Cell Containing a Naphthalocyanine/ReducedGraphene Oxide (FePc/RGO) Particulate Cathode and a High ElasticityPolymer-Protected Li Foil Anode

Particles of combined FePc/graphene sheets were obtained by ball-millinga mixture of FePc and RGO in a milling chamber for 30 minutes. Theresulting FePc/RGO mixture particles were potato-like in shape. Some ofthese mixture particles were encapsulated by a rotaxane network polymer(as described in Example 1) using the pan-coating procedure. Two lithiumcells were prepared, each containing a Li foil anode, a porousseparator, and a cathode layer of FePc/RGO particles (encapsulated orun-encapsulated).

The cycling behaviors of these 2 lithium cells are shown in FIG. 6,which indicates that the lithium-organic cell having a high-elasticitypolymer protection layer in the anode exhibits a significantly morestable cycling response. This protective polymer reduces or eliminatesthe continuing contact between the lithium metal and the electrolyte,yet the polymer layer itself remains in ionic contact with the lithiummetal and is permeable to lithium ions. This approach has significantlyincreased the cycle life of all lithium-organic batteries.

Example 5: Li—S Cells Containing an Anode-Protecting Layer and a CathodeContaining Sulfur-Impregnated Activated Carbon Particles

One way to combine sulfur with a conducting material (e.g.carbon/graphite particles) is to use a solution or melt mixing process.Highly porous activated carbon particles, chemically etched mesocarbonmicro-beads (activated MCMBs), and exfoliated graphite worms were mixedwith sulfur melt at 117-120° C. (slightly above the melting point of S,115.2° C.) for 10-60 minutes to obtain sulfur-impregnated carbonparticles.

FIG. 7 shows the cathode specific capacity values of two Li—S batterieshaving a cathode active material based on a S-impregnated activatedMCMB: one cell having a polyrotaxane-protected anode and the other onehaving no anode protection layer. It is clear that the rotaxane-basednetwork polymer layer (same as prepared in Example 2) implemented at theanode is highly beneficial to the cycle stability of the Li—S battery.

Example 6: Li—S Cells Containing an Anode-Protecting Layer and a CathodeContaining Sulfur-Coated Graphene Sheets

The cathode preparation procedure involves producing vapor of elementalsulfur, allowing deposition of S vapor on surfaces of single-layer orfew-layer graphene sheets. As a first step, the graphene sheets,suspended in a liquid medium (e.g. graphene oxide in water or graphenein NMP), were sprayed onto a substrate (e.g. glass surface) to form athin layer of graphene sheets. This thin layer of graphene was thenexposed to sublimation-generated physical vapor deposition. Sublimationof solid sulfur occurs at a temperature greater than 40° C., but asignificant and practically useful sublimation rate typically does notoccur until the temperature is above 100° C. We typically used 117-160°C. with a vapor deposition time of 10-120 minutes to deposit a thin filmof sulfur on graphene surface (sulfur thickness being approximately from1 nm to 10 nm). This thin layer of graphene having a thin film of sulfurdeposited thereon was then easily broken into pieces of S-coatedgraphene sheets using an air jet mill. These S-coated graphene sheetswere made into secondary particles of approximately 5-15 μm in diameter(e.g. via spray-drying) and then encapsulated by an UHMW PAN polymer.These encapsulated particulates were made into cathode electrodes usingthe conventional slurry coating procedure.

The cathode specific capacity values of two Li—S batteries having aS/graphene hybrid-based cathode active material and a lithium foil anodewith or without a high-elasticity polymer protection layer (same as inExample 3) are summarized in FIG. 8. These data have furtherdemonstrated the effectiveness of the high-elasticity polymer layerprotection approach.

Example 7: Effect of Lithium Ion-Conducting Additive in aHigh-Elasticity Polymer

A wide variety of lithium ion-conducting additives were added to severaldifferent polymer matrix materials to prepare anode protection layers.The lithium ion conductivity vales of the resulting polymer/salt complexmaterials are summarized in Table 1 below. We have discovered that thesepolymer composite materials are suitable anode-protecting layermaterials provided that their lithium ion conductivity at roomtemperature is no less than 10⁻⁶ S/cm. With these materials, lithiumions appear to be capable of readily diffusing through the protectivelayer having a thickness no greater than 1 μm. For thicker polymer films(e.g. 10 μm), a lithium ion conductivity at room temperature of thesehigh-elasticity polymers no less than 10⁻⁴ S/cm would be required.

TABLE 1 Lithium ion conductivity of various high-elasticity polymercomposite compositions as a lithium metal-protecting layer. SampleLithium-conducting % Rotaxane polymer No. additive network (1-2 μmthick) Li-ion conductivity (S/cm) E-1p Li₂CO₃ + (CH₂OCO₂Li)₂ 70-99% 1.5× 10⁻⁴ to 3.6 × 10⁻³ S/cm B1p LiF + LiOH + Li₂C₂O₄ 60-90% 4.5 × 10⁻⁵ to2.8 × 10⁻³ S/cm B2p LiF + HCOLi 80-99% 1.1 × 10⁻⁴ to 1.3 × 10⁻³ S/cm B3pLiOH 70-99% 8.9 × 10⁻⁴ to 1.2 × 10⁻² S/cm B4p Li₂CO₃ 70-99% 4.1 × 10⁻³to 9.2 × 10⁻³ S/cm B5p Li₂C₂O₄ 70-99% 8.4 × 10⁻⁴ to 1.4 × 10⁻² S/cm B6pLi₂CO₃ + LiOH 70-99% 1.4 × 10⁻³ to 1.6 × 10⁻² S/cm C1p LiClO₄ 70-99% 4.1× 10⁻⁴ to 2.1 × 10⁻³ S/cm C2p LiPF₆ 70-99% 2.2 × 10⁻⁴ to 6.1 × 10⁻³ S/cmC3p LiBF₄ 70-99% 1.3 × 10⁻⁴ to 1.6 × 10⁻³ S/cm C4p LiBOB + LiNO₃ 70-99%1.3 × 10⁻⁴ to 2.3 × 10⁻³ S/cm S1p Sulfonated polyaniline 85-99% 3.2 ×10⁻³ to 9.5 × 10⁻⁴ S/cm S2p Sulfonated SBR 85-99% 1.1 × 10⁻⁴ to 1.2 ×10⁻³ S/cm S3p Sulfonated PVDF 80-99% 1.6 × 10⁻⁴ to 1.2 × 10⁻⁴ S/cm S4pPolyethylene oxide 80-99% 4.1 × 10⁻⁴ to 3.2 × 103⁴ S/cm

Example 8: Cycle Stability of Various Rechargeable Lithium Battery Cells

In lithium-ion battery industry, it is a common practice to define thecycle life of a battery as the number of charge-discharge cycles thatthe battery suffers a 20% decay in capacity based on the initialcapacity measured after the required electrochemical formation.Summarized in Table 2 below are the cycle life data of a broad array ofbatteries featuring an anode with or without an anode-protecting polymerlayer.

TABLE 2 Cycle life data of various lithium secondary (rechargeable)batteries. Anode-protecting Initial capacity Cycle life Sample IDpolymer Type & % of cathode active material (mAh/g) (No. of cycles)CuCl₂-1e polyrotaxane 85% by wt. CuCl₂ particles (80 nm) + 536 1485 7%graphite + 8% binder CuCl₂-2e none 85% by wt. CuCl₂ particles (80 nm) +536 115 7% graphite + 8% binder BiF₃-1e none 85% by wt. BiFe₃particles + 7% 275 115 graphene + 8% binder BiF₃-2e Polyrotaxane + 85%by wt. BiFe₃ particles + 7% 276 1,340 50% ethylene oxide graphene + 8%binder Li₂MnSiO₄-1e polyrotaxane 85% C-coated Li₂MnSiO₄ + 7% 255 2,450CNT + 8% binder Li₂MnSiO₄-2e none 85% C-coated Li₂MnSiO₄ + 7% 252 543CNT + 8% binder Li₆C₆O₆-1e polyrotaxane + Li₆C₆O₆-graphene ball-milled441 1,450 20% polyanniline Li₆C₆O₆-2e none Li₆C₆O₆-graphene ball-milled438 116 MoS₂-1e polyrotaxane 85% MoS₂ + 8% graphite + binder 226 1,721MoS₂-2e none 85% MoS₂ + 8% graphite + binder 225 156

In conclusion, the high-elasticity polymer-based anode-protecting layerstrategy is surprisingly effective in alleviating the problems oflithium metal dendrite formation and lithium metal-electrolyte reactionsthat otherwise lead to rapid capacity decay and potentially internalshorting and explosion of the lithium secondary batteries. Thehigh-elasticity polymer is capable of expanding or shrinking congruentlyor conformably with the anode layer. This capability helps to maintain agood contact between the current collector (or the lithium film itself)and the protective layer, enabling uniform re-deposition of lithium ionswithout interruption.

We claim:
 1. A lithium secondary battery comprising a cathode, an anode,and a porous separator or electrolyte disposed between said cathode andsaid anode, wherein said anode comprises: a) an anode active layercontaining a layer of lithium or lithium alloy, in a form of a foil,coating, or multiple particles aggregated together, as an anode activematerial; and b) a thin layer of a high-elasticity polymer, disposedbetween the anode active layer and said porous separator or electrolyte;said polymer having a recoverable tensile strain from 2% to 1,500%, alithium ion conductivity no less than 10⁻⁶ S/cm at room temperature, anda thickness from 1 nm to 10 μm, wherein said high-elasticity polymercontains a polyrotaxane network having a rotaxane structure or apolyrotaxane structure at a crosslink point of said polyrotaxanenetwork.
 2. The lithium secondary battery of claim 1, wherein saidrotaxane structure or polyrotaxane structure is selected from rotaxane,a chemically modified rotaxane (rotaxane derivative), a polymer-graftedrotaxane, polyrotaxane, a co-polymer of polyrotaxane, a graft polymer ofpolyrotaxane, a polymer blend of polymer of polyrotaxane, a chemicallymodified polyrotaxane, or a combination thereof.
 3. The lithiumsecondary battery of claim 1, wherein said polyrotaxane network containsa polymer selected from polyethylene glycol, polypropylene glycol,polyethylene oxide, polypropylene oxide, poly (succinic acid), analiphatic polyester, or a combination thereof.
 4. The lithium secondarybattery of claim 1, wherein said polyrotaxane network contains a liquidthat permeates into spaces inside said network.
 5. The lithium secondarybattery of claim 1, wherein said high-elasticity polymer contains from0.1% to 50% by weight of a lithium ion-conducting additive dispersedtherein, or contains therein from 0.1% by weight to 10% by weight of areinforcement nanofilament selected from carbon nanotube, carbonnanofiber, graphene, or a combination thereof.
 6. The lithium secondarybattery of claim 1, wherein said high-elasticity polymer is mixed with alithium ion-conducting additive to form a composite wherein said lithiumion-conducting additive is dispersed in said high-elasticity polymer andis selected from Li₂CO₃, Li₂O, Li₂C₂O₄, LiOH, LiX, ROCO₂Li, HCOLi, ROLi,(ROCO₂Li)₂, (CH₂OCO₂Li)₂, Li₂S, Li—SO_(y), or a combination thereof,wherein X═F, Cl, I, or Br, R=a hydrocarbon group, 0<x≤1 and 1≤y≤4. 7.The lithium secondary battery of claim 1, wherein said high-elasticitypolymer further contains an electrically conducting material dispersedtherein and said electrically conducting material is selected from anelectron-conducting polymer, a metal particle or wire, a graphene sheet,a carbon fiber, a graphite fiber, a carbon nanofiber, a graphitenanofiber, a carbon nanotube, a graphite particle, an expanded graphiteflake, an acetylene black particle, or a combination thereof.
 8. Thelithium secondary battery of claim 7, wherein said electricallyconducting material has a thickness or diameter less than 100 nm.
 9. Thelithium secondary battery of claim 1, wherein said high-elasticitypolymer further contains a lithium salt dispersed therein and saidlithium salt is selected from lithium perchlorate, LiClO₄, lithiumhexafluorophosphate, LiPF₆, lithium borofluoride, LiBF₄, lithiumhexafluoroarsenide, LiAsF₆, lithium trifluoro-metasulfonate, LiCF₃SO₃,bis-trifluoromethyl sulfonylimide lithium, LiN(CF₃ SO₂)₂, lithiumbis(oxalato)borate, LiBOB, lithium oxalyldifluoroborate, LiBF₂C₂O₄,lithium oxalyldifluoroborate, LiBF₂C₂O₄, lithium nitrate, LiNO₃,Li-Fluoroalkyl-Phosphates, LiPF₃(CF₂CF₃)₃, lithiumbisperfluoro-ethysulfonylimide, LiBETI, lithiumbis(trifluoromethanesulphonyl)imide, lithium bis(fluorosulphonyl)imide,lithium trifluoromethanesulfonimide, LiTFSI, an ionic liquid-basedlithium salt, or a combination thereof.
 10. The lithium secondarybattery of claim 1, wherein said high-elasticity polymer is mixed withan electron-conducting polymer selected from polyaniline, polypyrrole,polythiophene, polyfuran, a bi-cyclic polymer, a sulfonated derivativethereof, or a combination thereof.
 11. The lithium secondary battery ofclaim 1, wherein the high-elasticity polymer forms a mixture or blendwith a lithium ion-conducting polymer selected from poly(ethylene oxide)(PEO), Polypropylene oxide (PPO), poly(acrylonitrile) (PAN), poly(methylmethacrylate) (PMMA), poly(vinylidene fluoride) (PVDF), Poly bis-methoxyethoxyethoxide-phosphazenex, Polyvinyl chloride, Polydimethylsiloxane,poly(vinylidene fluoride)-hexafluoropropylene (PVDF-HFP), a sulfonatedderivative thereof, or a combination thereof.
 12. The lithium secondarybattery of claim 1, wherein said thin layer of high-elasticity polymerhas a thickness from 1 nm to 1 μm.
 13. The lithium secondary battery ofclaim 1, wherein said thin layer of high-elasticity polymer has athickness less than 100 nm.
 14. The lithium secondary battery of claim1, wherein said thin layer of high-elasticity polymer has a thicknessless than 10 nm.
 15. The lithium secondary battery of claim 1, whereinsaid high-elasticity polymer has a lithium ion conductivity from 10⁻⁴S/cm to 10⁻² S/cm.
 16. The lithium secondary battery of claim 1, whereinsaid cathode active material is selected from an inorganic material, anorganic material, a polymeric material, or a combination thereof, andsaid inorganic material does not include sulfur or alkali metalpolysulfide.
 17. The lithium secondary battery of claim 16, wherein saidinorganic material is selected from a metal oxide, metal phosphate,metal silicide, metal selenide, transition metal sulfide, or acombination thereof.
 18. The lithium secondary battery of claim 16,wherein said inorganic material is selected from a lithium cobalt oxide,lithium nickel oxide, lithium manganese oxide, lithium vanadium oxide,lithium-mixed metal oxide, lithium iron phosphate, lithium manganesephosphate, lithium vanadium phosphate, lithium mixed metal phosphate,lithium metal silicide, or a combination thereof.
 19. The lithiumsecondary battery of claim 16, wherein said inorganic material isselected from a metal fluoride or metal chloride including the groupconsisting of CoF₃, MnF₃, FeF₃, VF₃, VOF₃, TiF₃, BiF₃, NiF₂, FeF₂, CuF₂,CuF, SnF₂, AgF, CuCl₂, FeCl₃, MnCl₂, and combinations thereof.
 20. Thelithium secondary battery of claim 16, wherein said inorganic materialis selected from a lithium transition metal silicate, denoted asLi₂MSiO₄ or Li₂Ma_(x)Mb_(y)SiO₄, wherein M and Ma are selected from Fe,Mn, Co, Ni, V, or VO; Mb is selected from Fe, Mn, Co, Ni, V, Ti, Al, B,Sn, or Bi; and x+y≤1.
 21. The lithium secondary battery of claim 16,wherein said inorganic material is selected from a transition metaldichalcogenide, a transition metal trichalcogenide, or a combinationthereof.
 22. The lithium secondary battery of claim 16, wherein saidinorganic material is selected from TiS₂, TaS₂, MoS₂, NbSe₃, MnO₂, CoO₂,an iron oxide, a vanadium oxide, or a combination thereof.
 23. Thelithium secondary battery of claim 17, wherein said metal oxide containsa vanadium oxide selected from the group consisting of VO₂, Li_(x)VO₂,V₂O₅, Li_(x)V₂O₅, V₃O₈, Li_(x)V₃O₈, Li_(x)V₃O₇, V₄O₉, Li_(x)V₄O₉, V₆O₁₃,Li_(x)V₆O₁₃, their doped versions, their derivatives, and combinationsthereof, wherein 0.1<x<5.
 24. The lithium secondary battery of claim 17,wherein said metal oxide or metal phosphate is selected from a layeredcompound LiMO₂, spinel compound LiM₂O₄, olivine compound LiMPO₄,silicate compound Li₂MSiO₄, Tavorite compound LiMPO₄F, borate compoundLiMBO₃, or a combination thereof, wherein M is a transition metal or amixture of multiple transition metals.
 25. The lithium secondary batteryof claim 17, wherein said inorganic material is selected from: (a)bismuth selenide or bismuth telluride, (b) transition metaldichalcogenide or trichalcogenide, (c) sulfide, selenide, or tellurideof niobium, zirconium, molybdenum, hafnium, tantalum, tungsten,titanium, cobalt, manganese, iron, nickel, or a transition metal; (d)boron nitride, or (e) a combination thereof.
 26. The lithium secondarybattery of claim 16, wherein said organic material or polymeric materialis selected from Poly(anthraquinonyl sulfide) (PAQS), a lithiumoxocarbon, 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA),poly(anthraquinonyl sulfide), pyrene-4,5,9,10-tetraone (PYT),polymer-bound PYT, Quino(triazene), redox-active organic material,Tetracyanoquinodimethane (TCNQ), tetracyanoethylene (TCNE),2,3,6,7,10,11-hexamethoxytriphenylene (HMTP), poly(5-amino-1,4-dyhydroxyanthraquinone) (PADAQ), phosphazene disulfide polymer ([(NPS₂)₃]n),lithiated 1,4,5,8-naphthalenetetraol formaldehyde polymer,Hexaazatrinaphtylene (HATN), Hexaazatriphenylene hexacarbonitrile(HAT(CN)₆), 5-Benzylidene hydantoin, Isatine lithium salt, Pyromelliticdiimide lithium salt, tetrahydroxy-p-benzoquinone derivatives (THQLi₄),N,N′-diphenyl-2,3,5,6-tetraketopiperazine (PHP),N,N′-diallyl-2,3,5,6-tetraketopiperazine (AP),N,N′-dipropyl-2,3,5,6-tetraketopiperazine (PRP), a thioether polymer, aquinone compound, 1,4-benzoquinone, 5,7,12,14-pentacenetetrone (PT),5-amino-2,3-dihydro-1,4-dyhydroxy anthraquinone (ADDAQ),5-amino-1,4-dyhydroxy anthraquinone (ADAQ), calixquinone, Li₄C₆O₆,Li₂C₆O₆, Li₆C₆O₆, or a combination thereof.
 27. The lithium secondarybattery of claim 26, wherein said thioether polymer is selected fromPoly[methanetetryl-tetra(thiomethylene)] (PMTTM),Poly(2,4-dithiopentanylene) (PDTP), a polymer containingPoly(ethene-1,1,2,2-tetrathiol) (PETT) as a main-chain thioetherpolymers, a side-chain thioether polymer having a main-chain consistingof conjugating aromatic moieties, and having a thioether side chain as apendant, Poly(2-phenyl-1,3-dithiolane) (PPDT),Poly(1,4-di(1,3-dithiolan-2-yl)benzene) (PDDTB),poly(tetrahydrobenzodithiophene) (PTHBDT),poly[1,2,4,5-tetrakis(propylthio)benzene] (PTKPTB, orpoly[3,4(ethylenedithio)thiophene] (PEDTT).
 28. The lithium secondarybattery of claim 16, wherein said organic material contains aphthalocyanine compound selected from copper phthalocyanine, zincphthalocyanine, tin phthalocyanine, iron phthalocyanine, leadphthalocyanine, nickel phthalocyanine, vanadyl phthalocyanine,fluorochromium phthalocyanine, magnesium phthalocyanine, manganousphthalocyanine, dilithium phthalocyanine, aluminum phthalocyaninechloride, cadmium phthalocyanine, chlorogallium phthalocyanine, cobaltphthalocyanine, silver phthalocyanine, a metal-free phthalocyanine, achemical derivative thereof, or a combination thereof.
 29. The lithiumsecondary battery of claim 1, wherein said cathode active material is ina form of nanoparticle, nanowire, nanofiber, nanotube, nanosheet,nanobelt, nanoribbon, nanodisc, nanoplatelet, or nanohorn having athickness or diameter from 0.5 nm to 100 nm.
 30. The lithium secondarybattery of claim 29, wherein said nanoparticle, nanowire, nanofiber,nanotube, nanosheet, nanobelt, nanoribbon, nanodisc, nanoplatelet, ornanohorn is coated with or embraced by a conductive protective coatingselected from a carbon material, graphene, electronically conductivepolymer, conductive metal oxide, or conductive metal coating.
 31. Thelithium secondary battery of claim 1, wherein said high-elasticitypolymer is a neat polymer having no additive or filler dispersedtherein.
 32. The lithium secondary battery of claim 1, which is alithium-sulfur battery, wherein said cathode comprises sulfur, asulfur-containing molecule, a sulfur-containing compound, a metalsulfide, a sulfur-carbon polymer, a lithium polysulfide, a sulfur/carbonhybrid or composite, a sulfur/graphite hybrid or composite, asulfur/graphene hybrid or composite, a sulfur-polymer compound, or acombination thereof.
 33. The lithium battery of claim 1, which is alithium metal battery, lithium-sulfur battery, lithium-selenium battery,or lithium-air battery.
 34. A method of manufacturing a lithium battery,said method comprising: (a) providing a cathode active material layerand an optional cathode current collector to support said cathode activematerial layer; (b) providing a lithium metal or lithium alloy foil orcoating and an optional anode current collector to support said foil orcoating; (c) providing an electrolyte and an optional separatorelectrically separating the anode and the cathode; and (d) providing ananode-protective layer of a high-elasticity polymer disposed betweensaid lithium metal or lithium alloy foil or coating and said electrolyteor separator, wherein said high-elasticity polymer has a recoverabletensile elastic strain from 2% to 1,500%, a lithium ion conductivity noless than 10⁻⁶ S/cm at room temperature, and a thickness from 0.5 nm to10 μm, wherein said high-elasticity polymer contains a polyrotaxanenetwork having a rotaxane structure or a polyrotaxane structure at acrosslink point of said polyrotaxane network.
 35. The method of claim34, wherein said rotaxane structure or polyrotaxane structure isselected from rotaxane, a chemically modified rotaxane (rotaxanederivative), a polymer-grafted rotaxane, polyrotaxane, a co-polymer ofpolyrotaxane, a graft polymer of polyrotaxane, a polymer blend ofpolymer of polyrotaxane, a chemically modified polyrotaxane, or acombination thereof.
 36. The method of claim 34, wherein saidpolyrotaxane network contains a polymer selected from polyethyleneglycol, polypropylene glycol, polyethylene oxide, polypropylene oxide,poly (succinic acid), an aliphatic polyester, or a combination thereof.37. The method of claim 34, wherein said polyrotaxane network contains aliquid that permeates into spaces inside said network
 38. The method ofclaim 34, wherein said high-elasticity polymer contains from 0.1% to 50%by weight of a lithium ion-conducting material dispersed therein, orcontains therein from 0.1% by weight to 10% by weight of a reinforcementnanofilament selected from carbon nanotube, carbon nanofiber, graphene,or a combination thereof.
 39. The method of claim 34, wherein saidhigh-elasticity polymer is mixed with a lithium ion-conducting additiveto form a composite wherein said lithium ion-conducting additive isdispersed in said high-elasticity polymer and is selected from Li₂CO₃,Li₂O, Li₂C₂O₄, LiOH, LiX, ROCO₂Li, HCOLi, ROLi, (ROCO₂Li)₂,(CH₂OCO₂Li)₂, Li₂S, Li_(x)SO_(y), or a combination thereof, wherein X═F,Cl, I, or Br, R=a hydrocarbon group, 0<x≤1 and 1≤y≤4.
 40. The method ofclaim 34, wherein said high-elasticity polymer further contains anelectrically conducting material dispersed therein and said electricallyconducting material is selected from an electron-conducting polymer, ametal particle or wire, a graphene sheet, a carbon fiber, a graphitefiber, a carbon nanofiber, a graphite nanofiber, a carbon nanotube, agraphite particle, an expanded graphite flake, an acetylene blackparticle, or a combination thereof.
 41. The method of claim 38, whereinsaid lithium ion-conducting material is dispersed in saidhigh-elasticity polymer and is selected from lithium perchlorate,LiClO₄, lithium hexafluorophosphate, LiPF₆, lithium borofluoride, LiBF₄,lithium hexafluoroarsenide, LiAsF₆, lithium trifluoro-metasulfonate,LiCF₃ SO₃, bis-trifluoromethyl sulfonylimide lithium, LiN(CF₃SO₂)₂,lithium bis(oxalato)borate, LiBOB, lithium oxalyldifluoroborate,LiBF₂C₂O₄, lithium oxalyldifluoroborate, LiBF₂C₂O₄, lithium nitrate,LiNO₃, Li-Fluoroalkyl-Phosphates, LiPF₃(CF₂CF₃)₃, lithiumbisperfluoro-ethysulfonylimide, LiBETI, lithiumbis(trifluoromethanesulphonyl)imide, lithium bis(fluorosulphonyl)imide,lithium trifluoromethanesulfonimide, LiTFSI, an ionic liquid-basedlithium salt, or a combination thereof.
 42. The method of claim 34,wherein said step of providing a lithium metal coating supported by ananode current collector is conducted during a first charge operation ofsaid battery.